In the manufacture of paper, plastic film or similar materials, highly undesirable web breakage occasionally occurs during machine operation and also at machine start-up. The web breakage is highly undesirable because the web has to be rethreaded prior to continuing manufacture. This can result in manufacturing delays. Web breakage in papermaking machines is even more undesirable because of the continuous running operation of these machines. In high-speed papermaking machines the paper continually moves through the machine at speeds up to 2000 meters/minute (m/min). Consequently, it is important for the rethreading of the web to be performed quickly and properly to reduce web wastage.
In practice, after a paper break, the web is threaded in stages through the papermaking machine by cutting a tail and threading the tail through the machine. Any part of this threading process that can be automated is advantageous because it reduces downtime associated with web breakage. The term tail refers to an edge piece cut into the traveling web by means of a cut into the web from the edge of the web and a continuous slit along the web which forms the "tail" or "edge piece" or "leader" of the paper web. This leader or tail may be blown or directed into the next portion of the paper making machine at which time the remainder of the web is severed so that the tail pulls the web through the next portion of the machine to be threaded. Typically the tail is anywhere from 5 to 20 centimeters in width compared to the remainder of the width of the web which can be in the order of 7 meters.
The rethreading process in a papermaking machine usually involves threading the tail from a first section of the machine such as, for example, a dewatering nip in the former, dryer or calender stack into a threading nip located downstream of the first section of the machine. The threading nip commonly comprises a pair of ropes entrained about pulleys between which the paper is threaded. The ropes move at or near the paper speed to pull the paper tail and draw the tail into the next section of the papermaking machine. The threading nip is normally located laterally offset from the normal through machine path of travel of the paper web so as not to interfere with the normal operation of the machine. However, the laterally offset positioning of the nip presents a major problem to rethreading a tail after web breakage. It is very difficult to laterally shift a tail of 5 to 20 centimeters in width moving at high speeds over runs of 0.5 to 6.5 meters consistently without twisting or folding the paper and thereby adversely effecting the planar integrity of the paper.
U.S. Pat. No. 4,136,808 issued Jan. 30, 1979 to Imants Reba discloses a web threading system for directing a paper tail laterally from one section of a papermaking machine and into threading nip ropes in a papermaking machine. The web threading system discloses redirection of the paper tail by the use of two Coanda nozzles angled relative to each other so that the paper tail changes direction as it passes over the nozzles. The nozzles direct the air stream or gas over two walls, or tray floors, which have two opposing upstanding sidewalls. The sidewalls redirect the air stream, tending to flow off the floor wall back over the floor wall. The paper tail, in theory, travels with the airflow over the floor walls; however, in practice the side walls are necessary to prevent the paper tail from falling over the sides of the floor wall. FIG. 1 of this patent has a first wall hinged relative to the second wall to redirect the paper tail up and into a threading nip. There is a lateral angular displacement shown in FIG. 2 between the first and second walls, which are angled relative to each other based on a radius formula. The first and second walls, however, are formed of two separate non-uniform separate tray structures that if laid flat do not provide a uniform structure. In practice, the disclosed web tray threading system does not provide a natural path of least resistance along which the paper follows into the threading nip. Consequently, the accuracy for determining the precise delivery location of the paper tail into the threading nip from this system cannot be predetermined with consistency. Further, the condition of the paper tail, that is its planar integrity, is most likely adversely affected because the paper does not move in a path of least resistance or one that seems natural for the paper tail to follow. The inaccuracy and inconsistency of the delivery of the paper tail into the threading nip is further affected by varying the web weight, moisture content and speed.